Field of the Invention
The present invention relates to a carbon film forming apparatus, a carbon film forming method, and a magnetic recording medium manufacturing method.
Priority is claimed on Japanese Patent Application No. 2013-273426, filed on Dec. 27, 2013, the content of which is incorporated herein by reference.
Description of Related Art
In recent years, in the field of magnetic recording media used in, for example, hard disk drives (HDDs), recording density has been remarkably improved and has been continuously increasing at a phenomenal rate of about 1.5 times a year. There are various techniques for improving the recording density. A technique which controls sliding characteristics between a magnetic head and a magnetic recording medium can be exemplified as one of the key technologies.
Protective films made of various materials have been proposed as the protective film of the magnetic recording medium. However, a carbon film has been mainly used, from the viewpoint of the overall performance, such as film formability and durability. In addition, for example, the hardness, density, dynamic friction coefficient, and compactness of the carbon film are very important since they are vividly reflected in the CSS (contact start-stop) characteristics or anticorrosion characteristics of the magnetic recording medium.
In order to improve the recording density of the magnetic recording medium, it is preferable to reduce the flying height of the magnetic head and to increase the number of rotations of the magnetic recording medium. Therefore, the protective film formed on the surface of the magnetic recording medium requires high sliding durability or flatness in order to cope with, for example, an accidental contact of the magnetic head. In addition, it is necessary to reduce the thickness of the protective film as much as possible, for example, to a thickness of 30 Å or less, in order to reduce the spacing loss between the magnetic recording medium and the magnetic head and to improve the recording density. There is a strong demand for a protective film which is smooth, thin, dense, and strong.
In addition, the carbon film used as the protective film of the magnetic recording medium is formed by, for example, a sputtering method, a CVD method, or an ion beam deposition method. Among these methods, when the carbon film is formed with a thickness of, for example, 100 Å or less by the sputtering method, the durability of the carbon film is insufficient. On the other hand, when the carbon film formed by the CVD method has low surface smoothness and a small thickness, the coverage of the surface of the magnetic recording medium is reduced, which may cause corrosion of the magnetic recording medium. In contrast, the ion beam deposition method is capable of forming a carbon film with high hardness, smoothness, and density, as compared to the sputtering method or the CVD method.
As a method of forming a carbon film using the ion beam deposition method, for example, a method has been proposed in which a deposition raw material gas is changed to plasma by discharge between a heated filament-shaped cathode and an anode in a film forming (deposition) chamber in a vacuum atmosphere and the resultant is then accelerated and collides with the surface of a substrate having a negative potential, thereby stably forming a carbon film with high hardness (see Japanese Unexamined Patent Application, First Publication No. 2000-226659).
A method has been proposed in which a magnet is provided around a carbon ion source and is rotated in a circumferential direction to form a carbon film with a uniform thickness and high hardness and density (see Japanese Unexamined Patent Application, First Publication No. 2011-65714).
In addition, a structure has been known in which magnets are provided around an ion source and on the rear side of the ion source (see Japanese Unexamined Patent Application, First Publication No. H6-176888). In the structure disclosed in Japanese Unexamined Patent Application, First Publication No. H6-176888, the magnet is not rotatable, but is fixed.
However, it is necessary to further reduce the thickness of the carbon film in order to further improve the recording density of the magnetic recording medium. It is necessary to manage the thickness of the carbon film on the basis of the thinnest portion of the carbon film formed on the surface of the magnetic recording medium in order to ensure the abrasion resistance or anticorrosion performance of the magnetic recording medium. Therefore, when the thickness distribution of the carbon film formed on the surface of the magnetic recording medium is not constant in the plane, it is difficult to reduce the thickness of the carbon film. In particular, in the method disclosed in Japanese Unexamined Patent Application, First Publication No. 2000-226659, since the filament, which is a carbon gas excitation source, has a shape that extends in one direction, the carbon film deposited on the surface of the substrate has a thickness distribution which depends on the shape of the filament.
In the method disclosed in Japanese Unexamined Patent Application, First Publication No. 2011-65714, the carbon film with high hardness and density can be formed with a uniform thickness over the wide range of the region on the surface (major surface) of the substrate. However, there is a large variation in thickness in the vicinity of the inner circumference and outer circumference of the substrate.